EDITORS:
The study, Features of Evolution and Expansion
of Modern Humans, Inferred from Genomewide
Microsatellite Markers, by Lev A. Zhivotovsky,
Noah A. Rosenberg and Marcus W. Feldman, appears in
the May 2003 edition of the American Journal of Human
Genetics. A copy can be obtained from Professor Marc
Feldman. His photo is available at http://newsphotos.stanford.edu
(slug: Humans_Feldman.jpg).

Scientists
use DNA fragments to trace the migration of modern
humans

Human
beings may have made their first journey out of
Africa as recently as 70,000 years ago, according to
a new study by geneticists from Stanford University
and the Russian Academy of Sciences. Writing in the American
Journal of Human Genetics, the researchers
estimate that the entire population of ancestral
humans at the time of the African expansion consisted
of only about 2,000 individuals.

"This
estimate does not preclude the presence of other
populations of Homo sapiens sapiens [modern
humans] in Africa, although it suggests that they
were probably isolated from one another genetically,
and that contemporary worldwide populations descend
from one or very few of those populations," said
Marcus W. Feldman, the Burnet C. and Mildred Finley
Wohlford Professor at Stanford and co-author of the
study.

The
small size of our ancestral population may explain
why there is so little genetic variability in human
DNA compared with that of chimpanzees and other
closely related species, Feldman added.

The
study, published in the May edition of the journal,
is based on research conducted in Feldman's Stanford
laboratory in collaboration with co-authors Lev A.
Zhivotovsky of the Russian Academy and former
Stanford graduate student Noah A. Rosenberg, now at
the University of Southern California.

"Our
results are consistent with the 'out-of-Africa'
theory, according to which a sub-Saharan African
ancestral population gave rise to all populations of
anatomically modern humans through a chain of
migrations to the Middle East, Europe, Asia, Oceania
and America," Feldman noted.

Ancient
roots

Since
all human beings have virtually identical DNA,
geneticists have to look for slight chemical
variations that distinguish one population from
another. One technique involves the use of
"microsatellites" -- short repetitive
fragments of DNA whose patterns of variation differ
among populations. Because microsatellites are passed
from generation to generation and have a high
mutation rate, they are a useful tool for estimating
when two populations diverged.

In their
study, the research team compared 377 microsatellite
markers in DNA collected from 1,056 individuals
representing 52 geographic sites in Africa, Eurasia
(the Middle East, Europe, Central and South Asia),
East Asia, Oceania and the Americas.

Statistical
analysis of the microsatellite data revealed a close
genetic relationship between two hunter-gatherer
populations in sub-Saharan Africa -- the Mbuti
pygmies of the Congo Basin and the Khoisan (or
"bushmen") of Botswana and Namibia. These
two populations "may represent the oldest branch
of modern humans studied here," the authors
concluded.

The data
revealed a genetic split between the ancestors of
these hunter-gatherer populations and the ancestors
of contemporary African farming people -- Bantu
speakers who inhabit many countries in southern
Africa. "This division occurred between 70,000
and 140,000 years ago and was followed by the
expansion out of Africa into Eurasia, Oceania, East
Asia and the Americas -- in that order," Feldman
said.

This
result is consistent with an earlier study in which
Feldman and others analyzed the Y chromosomes of more
than 1,000 men from 21 different populations. In that
study, the researchers concluded that the first human
migration from Africa may have occurred roughly
66,000 years ago.

Population
bottlenecks

The
research team also found that indigenous
hunter-gatherer populations in Africa, the Americas
and Oceania have experienced very little growth over
time. "Hunting and gathering could not support a
significant increase in population size,"
Feldman explained. "These populations probably
underwent severe bottlenecks during which their
numbers crashed -- possibly because of limited
resources, diseases and, in some cases, the effects
of long-distance migrations."

Unlike
hunter-gatherers, the ancestors of sub-Saharan
African farming populations appear to have
experienced a population expansion that started
around 35,000 years ago: "This increase in
population sizes might have been preceded by
technological innovations that led to an increase in
survival and then an increase in the overall birth
rate," the authors wrote. The peoples of Eurasia
and East Asia also show evidence of population
expansion starting about 25,000 years ago, they
added.

"The
exciting thing about these data is that they are
amenable to a combination of mathematical models and
statistical analyses that can help solve problems
that are important in paleontology, archaeology and
anthropology," Feldman concluded.

The
research was supported by grants from the National
Institutes of Health, the National Science Foundation
and the Russian Foundation for Basic Research.